Original article Scots pine susceptibility to attack by Tomicus piniperda (L) as related to pruning date and attack density B Långström C Hellqvist Swedish University of Agricultural Sciences, Division of Forest Entomology, S-776 98 Garpenberg, Sweden (Received 25 May 1992; accepted 23 November 1992) Summary &mdash; The susceptibility of young Scots pine to bark beetle attack was increased by pruning trees to a similar crown size = 10, 7 and 1 month(s) prior to beetle flight. Beetle population in the study area was high, and spontaneous attacks were expected to occur on the pruned trees. Half of the trees were baited with split pine bolts in order to attract more beetles to attack these trees. Thus, experimental trees carrying = one-third of their original foliage and with different vigour indices due to the pruning history were exposed to 2 levels of beetle attack. The pine shoot beetles preferentially attacked baited trees, whereas attack rates did not differ between pruning dates. Six wk after attack, beetle performance was better in trees pruned shortly before attack than in trees pruned earlier. Vig- our indices differed between the 2 treatments, but phloem starch, secondary resinosis (expressed as lesion size and resin acid content) and tree survival did not. Trees that eventually survived were sig- nificantly less attacked than those that died. but the 2 groups did not differ in tree characteristics (ex- cept in cambial electrical resistance). pine shoot beetles / Pinus sylvestris / beetle performance / defence reactions / host vitality Résumé &mdash; Susceptibilité du pin sylvestre aux attaques de Tomicus piniperda L en fonction de la date d’élagage et de la densité d’attaque. La susceptibilité de jeunes pins sylvestre aux atta- ques de scolytides a été accrue en élagant les arbres, de façon à ce que la taille de leur couronne soit comparable. Les élagages ont eu lieu environ 10, 7 et 1 mois avant le vol des insectes. Les ni- veaux de population dans la zone d’étude étaient élevés et des attaques spontanées étaient prévi- sibles sur les arbres élagués. Pour augmenter leur attractivité, la moitié des arbres ont été appâtés avec des rondins de pin. Ainsi, des arbres portant environ un tiers de leur feuillage d’origine, et ayant différents indices de vigueur à cause de l’élagage (tableau 1) ont été soumis à 2 niveaux d’atta- que. La moitié des arbres ont été coupés début juin, les autres fin août. T piniperda a attaqué de préférence les arbres appâtés (figs 1, 2) mais le taux d’attaque a été le même pour les différentes dates d’élagage (fig 1). Six semaines après les attaques, les arbres élagués le plus tardivement ren- fermaient plus d’insectes parents et plus de galeries contenant des larves que les arbres élagués précocement (tableau II). Les galeries maternelles étaient aussi significativement plus longues dans le premier cas (fig 1). Les arbres élagués environ 1 an avant l’attaque représentaient donc un maté- riel moins favorable pour les insectes. Les indices de vigueur différaient également entre les 2 traite- ments (tableau I), mais l’amidon présent dans le liber, la réaction secondaire (mesurée par la taille de la zone réactionnelle et son contenu en acides résiniques) et le taux de survie des arbres étaient semblables (fig 1, tableau III). La réaction de défense induite a avorté sur certains des arbres qui sup- portait une densité d’attaque supérieure à 200 galeries maternelles par m2 (fig 3). La longueur moyenne des galeries dépassait 40 mm (fig 4). Cependant, des arbres plus densément attaqués ont survécu. Chez les arbres résistants, les lésions occupaient au maximum 30% de la surface du phloème dans la partie basse du tronc (fig 5). Les arbres supposés survivants étaient significative- ment moins attaqués que les morts, mais leur taille, leur croissance et leur indice de vigueur étaient les mêmes (tableau IV). Cependant, la résistance électrique du cambium mesurée à la date de l’atta- que était significativement différente dans les 2 groupes, ce qui paraît illogique (tableau IV). Une des- cendance a été observée uniquement sur les arbres tués, avec un taux de multiplication inférieur à l’unité (tableau IV). Un début d’occlusion de l’aubier a été remarqué sur quelques arbres (potentielle- ment mourants ?) après 6 sem. L’aubier des arbres morts était fortement bleui, mais pas celui des arbres survivants. Tomicus piniperda / Pinus sylvestris / performance des insectes / réactions de défense / vitali- té de l’hôte INTRODUCTION In contrast to herbivores in general, most bark beetles attacking live trees need to kill their hosts in order to reproduce suc- cessfully. Consequently, host trees have evolved strong defence systems against bark beetles. Conifers counteract attacking bark beetles and their associated blue- stain fungi by a dual defence system based on primary resin which is exuded when resin ducts are severed, and by an induced secondary resinosis containing the aggressor in resin-soaked lesions (for an overview, see Christiansen et al, 1987). Successful colonisation by bark beetles occurs when the beetles can exhaust the defence system of the host trees by mas- sive synchronized attacks (Berryman et al, 1989; and references therein). Possession of aggregation pheromones as well as as- sociation with pathogenic blue-stain fungi seem to be typical features of tree-killing bark beetles (Christiansen et al, 1987; and references therein). As the resistance var- ies with host vitality, more beetles are needed to overwhelm the resistance of vigourous and fast-growing trees than less vital ones (Christiansen et al, 1987; and references therein). Thus, trees or stands may become susceptible to bark beetles as a result of reduced vitality and/or in- creased beetle populations, as exemplified by the concept of epidemic threshold (Ber- ryman, 1982). In Europe, Tomicus piniperda (L) (Col Scolytidae) is the most important bark bee- tle attacking Scots pine (for references, see eg Escherich, 1923; Postner, 1974; Långström, 1983). In northern Europe, however, T piniperda is seldom capable of successfully mass attacking living pine trees, whereas in more southerly areas it has been reported to kill trees from time to time (for references, see Långström and Hellqvist, 1991). This difference in beetle aggressiveness or host susceptibility trig- gered our interest in studying this pest- host relation under our conditions. So far, we have found that even low- vigour Scots pines that were additionally weakened by pruning have a remarkable resistance to induced attacks by T piniper- da (Långström and Hellqvist, 1988). Trees responded with vigourous induced defence reactions, enclosing the beetles in resin- soaked lesions. Typically, trees that failed to resist attacks accumulated less resin ac- ids in the lesions and depleted their starch reserves in the phloem (Långström et al, 1992). Two species of blue-stain fungi, Leptographium wingfieldii Morelet and Ophiostoma minus (Hedgc) H et P Syd, were frequently isolated from the sapwood of killed trees (Solheim and Långström, 1991). The same species have been found to be associated with T piniperda in France (Lieutier et al, 1989b). Thus, the interaction between the beetle, its fungi and Scots pine seems to be similar in Sweden and in France (Lieutier et al, 1988; 1989a; Lieuti- er, in press). The physiological mechanisms underly- ing host resistance to bark beetles are poorly understood. Carbohydrates, being both an energy source and raw material for the defence chemistry, may be important (Christiansen et al, 1987; and references therein); especially the tree’s capacity to translocate carbohydrates to the area un- der attack (Christiansen and Ericsson, 1986; Miller and Berryman, 1986; Långström et al, 1992). Hence, manipula- tion of needle biomass and tree vitality (de- fined as vigour index sensu Waring and Pitman, 1985) should affect the tree’s de- fence capacity in a predictable way. Our previous studies also showed that pruned trees succumbed more frequently to beetle attack than unpruned trees, but as the for- mer were also subject to more attacks, we could not separate the effect of attack den- sity on the induced defence reaction from that of host tree vigour. Thus, in the present study, we com- pared the susceptibility of weakened trees with a similar needle biomass but different vigour indices (ie a similar capacity to pro- duce carbohydrates, but different growth efficiency) to induced attacks by pine shoot beetles. By relating beetle performance and defence reactions to tree characteris- tics, we attempted to identify factors typical for resistant trees, as well as critical attack levels for trees of different vitality. MATERIAL AND METHODS Field work The experimental site was a = 30-yr-old pure pine stand at Norrsundet in Gästrikland, Central Sweden (= 61 °N lat, 16 °C long). The pine trees displayed misshapen crowns due to intensive shoot-feeding by pine shoot beetles over many years, and were obviously not in good condition (see also Långström and Hellqvist, 1988; Långström et al, 1992). In order to create a tree population with re- duced but similar capacity for carbohydrate pro- duction despite different vigour indices, trees were pruned to similar needle biomass on 3 oc- casions prior to beetle attack. In June 1988, 60 similar-looking (diameter, height and crown size) pine trees were selected for this pruning experi- ment in the low-vigour stand described above. Twenty of these trees were pruned on 21 June (after beetle flight in 1988), 9 September 1988 and 9 March (prior to beetle flight in 1989), re- spectively, leaving the 7-8 uppermost whorls in- tact (table I). As the beetle population was high in the area, beetle attacks were expected to occur on the pruned trees (cf Långström and Hellqvist, 1988). In order to induce a higher level of beetle attack, half of the trees (10 in each treatment) were furnished with split bolts of fresh pine wood to enhance host attraction to the beetles (Långström and Hellqvist, 1988). This baiting was carried out on 9 March 1989, but as beetle flight started later than expected, all bait-bolts were replaced with new bolts on 13 April, when flying beetles were observed in the stand. Judg- ing from meteorological data, that day was prob- ably the first day of Tomicus flight in the area. In an attempt to measure tree vitality at the time of beetle attack, we measured the cambial electrical resistance (CER) of the inner bark with a Shigometer, especially developed for this pur- pose (for a technical description and references, see Lindberg and Johansson, 1989). This tech- nique has been used in different contexts for de- scribing tree vitality (see eg Piene et al, 1984a, 1984b; Matson et al, 1987), and also in bark beetle studies, but with contradictory results (Christiansen, 1981; Lieutier and Ferrell, 1988). CER readings were taken from experimental trees on 2 occasions: 11 April (all trees) and 13 April (baited trees only). Readings were taken in early afternoon, and the ambient temperature was recorded every 30 min. From each tree, 2 readings were taken with the probes inserted vertically into the bark at opposite sides of the stem at breast height. Uncorrected readings were used since ambient temperature was stable during the procedure and close to the standard 15 °C. Half of the pruned trees were felled on 1 June (when beetle tunelling was still in progress and developed lesions were expected to be found; cf Långström et al, 1992), and the re- maining pruned trees on 24 August 1989 (when the brood had emerged and trees had either died or survived). After felling, tree length, crown length, annual height growth back to 1983, crown fresh weight (ie all live branches), and the number of live whorls were recorded. The trees were classified as surviving, survival uncertain, dying or dead, according to the ap- pearance of the foliage and the inner bark. A stem disc was sawn at breast height, and the border between the translucent sapwood and opaque heartwood marked immediately. All stems (up to the first living whorl at = 3 m height) were transported to the laboratory within 24 h, and cold-stored at +2 °C until the next day. Laboratory procedures On the day after felling, the stems were cut at 20 and 30 cm stem height. The lower sections were discarded, and the upper 10-cm pieces were placed in trays with a few cm of a water suspension of Fast Green (0.25 g per 1 I water; Parmeter et al, 1989) and were allowed to take up the dye for 24 h at room temperature. Then new surfaces were cut = 5 cm above the lower end of the bolts and the presence of unstained non-conducting sapwood and heartwood was delineated. After cutting the stem in sections, the bolts between 30-80 and 130-180 cm stem heights were immediately frozen, the 80-130-cm sec- tion taken for isolation of fungi from beetle gal- leries and sapwood (Solheim and Långström, 1991), and the remaining sections up to live crown were cold-stored until analysed. Before removing the bark on the 30-80-cm stem section (and section 130-180 cm, if T mi- nor (Hart) was present), all exit holes of the emerging new brood of pine shoot beetles were counted (not applicable for June-felled trees). If galleries of T minor were present under the bark, the exit holes of this species were counted on the wood surface, the difference between the 2 counts then being attributable to T piniperda. As the bark was relatively thin, no correction was made for the few beetles emerging through old exit holes (cf Salonen, 1973). The presence of blue-stain on the cut bolt ends was noted in 10% area classes. For the first 20 galleries encountered of each beetle species after bark removal, the following were recorded: total gallery length, length of le- sion tip ahead of the gallery tip, total lesion length, presence of parent beetle(s), eggs, lar- vae or pupae in the gallery; then the lesions sur- rounding the galleries were delineated on trans- parent film; finally, the lesions were cut out along the lesion periphery and refrozen for later chemical analyses (June-felled tree only). All ad- ditional galleries as well as those found on the other stem sections (including that taken for iso- lation of fungi) were counted, separating beetle species and attack attempts (< 1 cm in gallery length) from longer egg galleries (> 1 cm in length). For trees felled in June, additional phloem samples were taken from an unaffected part of the stem (> 10 cm from the nearest lesion) for later analyses of resin acids and starch, and from phloem adjoining lesions for starch analy- ses; all samples were refrozen as the lesion samples mentioned above. The discs taken at breast height were pol- ished, and annual ring widths were measured with 0.01 mm accuracy along 2 opposite radii. Radial growth, basal area growth, vigour index (ie the cross-sectional area of a given annual ring (or rings) in percent of the total sapwood area; see Waring and Pitman, 1980; for a dis- cussion of the underlying physiological assump- tions, see Waring and Pitman, 1985), and sap- wood percentage were calculated and used as expressions of tree vitality prior to beetle attack (table I). Lesion areas were calculated as lesion length by mean lesion width (obtained from measurements of lesion widths for every cm in length from the drawings on transparent film). A net lesion area was obtained by subtracting the egg gallery area (calculated as gallery length x 2 mm average egg gallery width). Knowing the attack density and the mean lesion area, the to- tal lesion area per m2 inner bark could be calcu- lated. Chemical analyses Inner bark samples were pooled within each pruning date into 3 attack density classes (see below) prior to analysing resin acids and starch as previously described by Långström et al (1992). Statistics Data were analysed using the SAS statistical program package (SAS, 1987). Treatment means were compared by analyses of variance followed by Tukey’s test for multiple compari- sons, or by 2-way ANOVAs (Zar, 1984). Pairs of means were tested with Student’s t-test, correct- ing for unequal variances when appropriate (Zar, 1984). The resin acid composition in the samples was analysed by principal component analysis (PCA). Relationships between vari- ables were analysed using correlation coeffi- cients and stepwise linear regressions were computed in order to explain the variation. RESULTS Tree vigour Tree diameter, height and number of re- maining whorls after pruning were similar for experimental trees of the 3 pruning dates (table I; ANOVA followed by Tukey’s test for multiple comparisons). Height growth, crown length, ring widths and sev- eral other expressions of tree vigour were significantly lower for the trees pruned in June 1988 than for trees pruned in April 1989, and intermediate for trees pruned in August 1988 (table I). However, there was no difference in cambial electrical resis- tance. Beetle performance Beetle attack All but one of the 60 trees included in the study were attacked by T piniperda and 13 trees were also attacked by T minor. The attack density of the latter species was negligible (maximum of 5 galleries on the tree attacked most); hence no further at- tention will be paid to T minor in this study. Since no other bark-living insects were found on the stem sections in any num- bers, T piniperda (and its associated blue- stain fungi) was the major challenge of the tree’s defensive capacity. The attack density of T piniperda on the lower stem (0.3-0.8 m) did not differ signif- icantly between pruning dates (fig 1; 2-way ANOVA; data for the 2 sampling dates were pooled, as they did not differ). As ex- pected, the attack density was higher on baited trees than on unbaited ones (see also figure 2). The attack density on the lower stem was well correlated with the total number of egg galleries on the whole trunk ex- ploited by the beetles (fig 2). Although baited trees were clearly more attacked, there was a great overlap between the 2 groups. Gallery construction Egg galleries were significantly shorter in trees pruned in June 1988 than in those pruned in March 1989 (fig 1; ANOVA fol- lowed by Tukey’s test for multiple compari- sons), indicating more persistent oviposi- tion attempts in the latter than the former trees. Correspondingly, the percentage of galleries < 1 cm, ie failed attack attempts rather than true galleries, differed clearly between tree groups (fig 1). At attack densities < 200 egg galleries per m2, mean egg gallery length remained short, indicating failure in establishing a brood (fig 3). This was true for both batch- es of trees, ie trees felled in June as well as in August. At higher attack densities, gallery lengths were also similar between the 2 groups of trees, indicating that full gallery length had been reached by 1 June. It is noteworthy that some of the sur- viving trees had gallery lengths similar to those that eventually died. Beetle behaviour and brood development Since attack densities on baited and un- baited trees were overlapping (cf fig 2), trees were regrouped in 3 attack density classes within each pruning date (< 150, 151-300, > 300 egg galleries per m2, re- spectively) regardless of whether they had been baited or not, before further analyses of beetle behaviour and defence chemistry were made. By 1 June, all trees with a low attack density were abandoned by the parent beetles and no larvae had hatched in the galleries (table II), regardless of pruning date. Presence of parent beetles as well as the percentage of galleries with devel- oping brood was higher in severely than in intermediately attacked trees. For the trees attacked most, these percentages in- creased from the oldest to the latest prun- ing date. Thus, attack density had a large influence on the probability for successful colonisation, and the beetles seemed to do better on trees pruned shortly before than long before attack (successful brood devel- opment occurred only in trees that were eventually killed by the attacks; see be- low). [...]... IUFRO Working Party and XVII Int Congr Entomol Symp, Vancouver, BC, Canada, July 4 1988, 121-133 Långström B, Hellqvist C, Ericsson A, Gref R (1992) Induced defence reaction in Scots pine following stem attacks by Tomicus piniperda Ecography 15, 318-327 Långström B, Hellqvist C (1993) Induced and spontaneous attacks by Tomicus piniperda and T minor on young Scots pine trees J Appl Entomol 115, 25-36 Larsson... Scolytidae) Acta For Fenn 127,72 SAS Institute Inc (1987) SAS/STAT Guide for Personal Computers Version 6 ed, pp 1028 Schroeder LM (1990) Duct resin flow in Scots pine in relation to the attack of the bark beetle Tomicus piniperda (L) (Col, Scolytidae) J Appl Entomol 109, 105-112 Solheim H, Långström B (1991) Blue-stain fungi associated with Tomicus piniperda in Sweden and preliminary observations on... in the lesion phloem as compared to unaffected control bark samples (table III) No clear differences could be seen in the total amounts of resin acids either between pruning dates or attack density classes As the resin acid composition did not differ between trees of different pruning dates and attack density classes, data are not shown The principal component analysis of lesion and control samples... seems to be mainly quantitative Comparison between and dead trees Tree surviving mortality and characteristics Ten of the 30 trees felled in August 1989 were classified as dead, and of these 4, 2 and 4 trees had been pruned in June 1988, September 1988 and March 1989, respectively As 6 of the dead trees had been baited and 4 not, tree mortality was not clearly linked to either baiting or pruning date. .. presence as well as brood development was lower in trees pruned in June 1988 than in other trees Correspondingly the frequencies of failed attacks (ie short galleries) tended to be higher and mean gallery lengths shorter in trees pruned in June 1988 than in trees pruned closer to the beetle attack, despite similar attack densities (both felling dates; cf fig 1).Thus, the trees pruned 1 yr prior to beetle attack. .. 56, 277-283 Larsson S, Oren R, Waring RH, Barrett JW (1983) Attacks of mountain pine beetle as related to tree vigour of ponderosa pine For Sci 29, 395-402 Lieutier F (1993) Hypothetical working mechanisms of the induced defence reaction of conifers to bark beetles and their associated Ophiostoma In: Int Symp Taxonoy and Biology of the Ophiostomales Bad Windshein, Germany, 21-24 August 1990 (in press)... grandis inoculated with Trichosporum symbioticum in northeastern Oregon US Dept Agric, For Serv, Pacific Northwest Res Stat, Res Note PNW-RN-489, pp 12 Långström B (1983) Life cycles and shootfeeding of pine shoot beetles Stud For Suec 163,29 Långström B, Hellqvist C (1988) Scots pine resistance against Tomicus piniperda as related to tree vitality and attack density In: Integrated Control of Scolytid Bark... in a similar way The attack density of T piniperda was comparable to that found in other studies in the same area (Långström and Hellqvist, 1988, (1993); Långström et al, = sion and gallery length Thus, the linear relationship between gallery expansion and lesion formation observed by Lieutier et al (1988), was valid only until the defence system started to collapse when an increasing number of beetles... the other hand, removal of two-thirds of the foliage resulted in onethird of the trees left to grow over the summer being killed by beetle attack This would hardly have happened with trees carrying intact foliage (cf Långström et al, 1992; and references therein) Thus, the pruning treatment substantially increased the susceptibility of the trees to beetle attack The reduced defence capacity was also reflected... Hence, further studies are needed to develop methods of identifying and measur- ing even short-term changes in host susceptibility to bark beetles The physical or chemical properties of the phloem could be trait to pursue, as indicated by the measurements of cambial electrical resistance, a despite conflicting results ACKNOWLEDGMENTS We thank Stora AB for permission to work on their premises, KSLA for . Original article Scots pine susceptibility to attack by Tomicus piniperda (L) as related to pruning date and attack density B Långström C Hellqvist Swedish. Scots pine following stem attacks by Tomicus pini- perda. Ecography 15, 318-327 Långström B, Hellqvist C (1993) Induced and spontaneous attacks by Tomicus piniperda and T. mortality was not clearly linked to either baiting or prun- ing date. Six of the dead trees had been attacked by T piniperda alone, and 4 trees by both Tomicus species. The